The present invention relates to hydraulic drive systems, for vehicles such as bicycles for example, and more particularly to bicycles having a hydraulic variable speed drive mechanism.
In the past, bicycles have typically been driven by chain and sprocket mechanisms. In recent years with the addition of one or more drive sprockets on the crank and a cluster of sprockets on the rear wheel, the bicyclist has been able to select any one of a number of gear ratios by means of a manual shift mechanism which "derails" the chain from one sprocket and moves it to another. Such a shift mechanism can frequently become subject to erroneous operation or result, and can be a clumsy mechanism to operate, the bicyclist sometimes moving the shift mechanism to select a desired gear ratio only to find that the chain has missed the desired gear, sometimes engaging an undesired one. Furthermore, the drive sprocket/sprocket cluster arrangement does not provide a constant pedal pressure or pedal rotation rate over a wide range of bicycle speeds. In other words, the pedal rotation rate and pedal pressure will vary within a given range for any selected gear ratio, from a low rotation rate where considerable pedal pressure may be required to maintain the bicycle's speed to a high pedal rotation rate where the legs have to be moved very rapidly, at a very tiring pace, to maintain the desired speed. Between these extremes, for any particular road condition encountered there is no doubt some gear ratio which will produce a reasonably optimum pedal rotation rate which corresponds to an optimum leg speed and strength for a given individual, but the rider must frequently search for those optimum conditions by shifting the "derailer" mechanism. It can clearly be seen, therefore, that a constant pedal rotation rate and pedal pressure is not maintained across a wide range of bicycle speeds by even the multiple-geared chain and sprocket drive systems.
At least a small number of bicycles have been proposed heretofore employing hydraulic drive mechanisms. In particular, U.S. Pat. No. 4,290,621 issued to Davey discloses a hydraulic multi-speed drive mechanism which employs a plurality of gear pumps and gear motors to drive the bicycle. The drive mechanism of this patent has many of the same disadvantages as the chain/sprocket mechanism discussed above, since it employs a plurality of manually operated valves to adjust the pumping of the hydraulic fluid and the rate at which the hydraulic motor is driven. While it may theoretically be possible for the user of the aforementioned device to continuously adjust the valves to produce a constant pedal pressure and pedal rotation rate for a wide range of bicycle speeds, to do so would be extremely inconvenient and the probable result would be that the bicyclist would not in fact achieve a constant pedal pressure and rotation rate over a wide range of bicycle speeds.
The device of the Davey patent and other prior proposals thus leave a great deal to be desired from a practical standpoint, and do not fulfill much of the promise which a hydraulic drive system really has for bicycles, since they do not provide the essential element of selectably variable "mechanical advantage", particularly by way of mechanisms or means which do not require constant vigilence and manipulation by the rider.